Short-circuiting this object is feasible, but requires some refactoring in NiftMapsMasker, to expose an internal function that can take masked data as input (sort of reminds me of some functions that I wrote a few weeks ago and which were eliminated ;-) ). 

A better organization of the code is always a good thing. The question is: should we do this now? Might not it be a good idea to release a mildly-optimized implementation that works to get feedback first?